1. Field of the Invention
The present invention relates to an optical coupler, especially to a KBBF family nonlinear optical crystal-prism coupler.
2. Description of the Related Art
Two or more optical components are generally bonded together in an optical system. Generally, there are three following methods: optical adhesive bonding, optical direct bonding and diffusion bonding. The optical adhesive bonding bonds two polished mirror surfaces with optical adhesive. This technique is simple and can be used to bond different optical materials. There is a disadvantage that any intermediate bonding material may form flaw and stains on the surfaces, affecting the quality of devices. Furthermore, it is difficult to achieve an optical adhesive of which refractive index completely consists with that of the optical components, especially when the two optical components are different optical materials. Light unavoidably suffers Fresnel reflection and thus loses light flux while travelling through interfaces of the components and adhesive. Finally, a bonding force of optical adhesive is insufficient. Especially, the optical adhesive is liable to be softened, deformed and even crack under heat in a laser system. Therefore, the optical adhesive is easy to cause optical damage under a high laser power so as to affect application.
The second method is the optical direct bonding. This is a technique to bond optical components with ultra-smooth surfaces without aiding of any adhesive and also referred to as “optical contact”. The principle is that two sufficiently smooth surfaces are close to each other and the molecules of them are closely adjacent to each other, so that the smooth surfaces form a small distance therebetween and attract each other due to an electromagnetic interaction. Smoother the surfaces are, then closer the molecules are, and stronger the interaction is. A resultant force of this molecular interaction is even stronger than the adhesive force of optical adhesive of the first method. Optical direct bonding is widely applicable for the bonding between the same two optical materials. However, if the two optical components for bonding are of different optical materials or of the same optical material with different surface structures, inconsistent expansion will result in the separation of the two components due to the difference in thermal expansion coefficient when the interface is heated. This phenomenon is particularly common in a high-power laser system.
In view of the above, optical direct bonding between different materials is generally limited to be used in a low-power laser system. For example, a 532 nm green laser crystal assembly, which is made of a current common laser crystal (Nd: YVO4) and a nonlinear frequency doubling crystal KTP by optical direct bonding, can generate more than 70 mW green laser when the laser diode pump has a power of 500 mW.
The third method is high-temperature diffuse bonding. This method performs the common optical contact followed by high-temperature heat treatment so that the two optical components occur high-temperature diffusion of atoms near an interface thereof so as to enhance the bonding force. This method is generally used for bonding optical materials of the same type. For example, undoped YAG crystals optically contact with two ends of a doped YAG crystal laser bar by bonding through high-temperature treatment so as to improve the thermal performance of the laser bar.
KBBF family crystal is a unique nonlinear optical material that can achieve deep ultraviolet harmonic light output by a direct frequency doubling method. A coupling technique for optically contacting a nonlinear optical crystal with prisms (PCT) and a device using the coupling technique have been invented (ZL 01115313.X; U.S. Pat. No. 6,859,305B2; Japanese Patent 4074124), in order to solve a difficulty in cutting a KBBF family crystal with a layered structure in a phase matching direction. The coupling technique makes the crystal to achieve practical and precise deep ultraviolet frequency doubling functions, such as high efficiency, high light beam quality, wide tuning etc.
In current optical processing field, a KBBF family crystal and CaF2 crystal prisms or quartz glass prisms are coupled by common optical contact to fabricate a KBBF family crystal-prism coupler. The crystal and the prisms are of different materials, the binding force therebetween is insufficient, and thermal properties such as thermal expansion coefficients of two materials at the interface, etc. are different. Therefore, the two optical components may suffer different thermal expansions to be separated or appear interface damage. Especially, a high-power laser system has high laser power, long duration, poor heat dissipation at the interface, and is generally damaged primarily at the interface due to over-high temperature. As a result, laser beam quality degrades, and the two interfaces may separate from each other to cause the high-power laser system disable. In addition, to, it is advantageous to bond multiple KBBF family crystals by optical contact bonding in order to increase the effective path length of light. Further, compose of the KBBF family crystal is complex and there are many atomic species. Accordingly, the KBBF family crystal has complex thermal property, which is also called as poor optical contact ability. In the high-power laser system, the interfaces are also damaged at first and the two interfaces separate from each other if it is worse.